1. Background of the Invention
The invention relates generally to the removal of vapor phase contaminants from a gas stream. More specifically, the invention is directed to a method and apparatus for the removal of vapor phase contaminants, such as mercury, from the flue gas of a combustion system.
2. Description of Related Art
The emission of trace metals from utility power plants is an important. In particular, special attention has been given to trace contaminants, including, for example, mercury (Hg), in terms of their release into the environment and corresponding impacts on the environment. Generally, trace contaminants include those vaporous chemical species present in relatively low concentrations in a given gas stream as well as solid particulate matter. For example, mercury is present in flue gas from a fossil-fuel-fired combustion system in very low concentrations (<1 ppb) and forms a number of volatile compounds that are difficult to remove. Specially designed and costly emissions-control systems are required to effectively capture these trace amounts of mercury.
Several approaches have previously been adopted for removing mercury from gas streams. These techniques include passing the gas stream through a fixed or fluidized sorbent filter or structure or using a wet scrubbing system. Approaches using fixed bed technologies normally pass the mercury containing gas through a bed consisting of sorbent particles or through various structures such as honeycombs, screens, or fibers that are coated with a sorbent. Common sorbents include activated carbon and noble metals such as gold and silver. In many cases where noble metals are used, the structure is coated with the noble metal sorbent while the support underneath is made of ceramic or metallic materials. The sorbents in these fixed structures can be periodically regenerated by heating the structure and driving off the adsorbed mercury (see, for example, U.S. Pat. Nos. 5,409,522 and 5,419,884, which are incorporated by reference herein in their entireties). The mercury driven off can then be recovered or removed separately.
However, in regenerating the sorbent in such fixed bed systems, the bed must be taken off-line periodically. This necessitates that a second bed be used and remain on-line while the first one is regenerating. In addition, the beds need to be located downstream of a primary particulate collection device to remove all of the solid suspended particles in the gas stream and to avoid pluggage. These fixed bed systems also require significant space since they need to remove vapor phase contaminants, such as mercury, for long periods of time without having to be replaced or regenerated, and they are very difficult to retrofit into existing systems, such as into the ductwork of power plants, without major modifications and high pressure drop penalties (e.g., 10-30 inches of water).
U.S. Pat. Nos. 5,948,143 and 6,136,072, which are incorporated by reference herein in their entireties, describe concepts that addressed some of these problems through the use of porous tubes and plates that can be regenerated and cleaned while in the presence of flue gas containing particles. These porous tubes and plates are cleaned by a series of back pulses across their walls. However, the fabrication of porous tubes and plates is complex and relatively expensive. The tubes and plates are also heavy and difficult to install and heat due to the thick wall requirements.
Therefore, a need remains for a cost-effective method and apparatus for removing trace contaminants, in particular mercury, from gas streams, including, for example, the flue gas of a coal-fired combustion system. In addition, there is a need for an improved process and apparatus for removing such contaminants that can be easily retrofitted into an existing combustion system.